1. Field of the Invention
The present invention relates to a bias circuit for a differential amplifier or a mixer circuit applied to an instrument processing a high frequency signal, such as a portable telephone.
2. Description of the Related Art
FIG. 6 shows a prior art differential amplifier 11. The amplifier 11 includes transistors Q1 to Q3, resistors R1 to R6, capacitors C1 and C2, and a constant-current source I1. The amount of current flowing through the transistors Q1 and Q2 depends upon the resistor R1. The transistors Q1 and Q2 are supplied with a bias voltage from a bias circuit constituted by the constant-current source I1, transistor Q3, and resistors R2, R5 and R6. When the level of an input signal IN supplied to the base of the transistor Q1 is low, a fixed amount of current flows through the differential amplifier 11. As the input signal IN increases in level, the current flowing through the amplifier 11 increases because of a nonlinear operation of the transistors Q1 and Q2. If the level of the input signal 11 increases further, the emitter potentials of the transistors Q1 and Q2 are clamped by the ground level, and the current flowing through the differential amplifier 11 increases abruptly.
FIG. 7 shows a prior art double-balance mixer circuit 12. The mixer circuit 12 includes transistors Q11 to Q17, resistors R11 to R16, capacitors C11 and C12, and a constant-current source I11. The transistors Q11 and Q12 constituting a first differential amplifier, serve as a stage supplied with an input signal IN. The emitters of the transistors Q14 and Q15, which constitute a second differential amplifier, are connected to the collector of the transistor Q11, and the emitters of the transistors Q16 and Q17, which constitute a third differential amplifier, are connected to the collector of the transistor Q12. The bases of the transistors Q14 to Q17 are supplied with a local oscillation signal LOC from a local oscillator (not shown). The transistors Q11 and Q12 are supplied with a bias voltage from a bias circuit constituted by the constant-current source Ill, transistor Q13, and resistors R12, R15 and R16. If the input signal IN increases in level, the current flowing through the mixer circuit 12 driven by the bias circuit increases, and the characteristic of the mixer circuit deteriorates, as in the case of the differential amplifier shown in FIG. 6.
For example, in portable telephones, when their antennas are put into contact with each other, a 10 dBm or more high-level signal is likely to enter at the antennas. This type of portable telephone is driven by a battery, and an integrated circuit incorporated into the telephone and having a differential amplifier or a mixer circuit is operated at a voltage of, for example, 2.2 V and 5.5 V. For this reason, when a high-level input signal of 2 V.sub.p-p or more is received, the transistors Q1 and Q2 shown in FIG. 6 and the transistors Q11 and Q12 shown in FIG. 7 are operated in a saturation region, and the S/N (signal-to-noise) ratio is easy to deteriorate. In the mixer circuit, when the transistors Q11 and Q12 are operated in the saturation region, the input signal is likely to leak toward the local oscillator.
If the resistor R1 of FIG. 6 and the resistor R11 of FIG. 7 are replaced with constant-current sources, an operation current can be prevented from increasing in the differential amplifier and mixer circuit even when a high-level signal is input to the differential amplifier and mixer circuit. In general, however, the noise generated from the constant-current source is greater than the noise generated from the resistor R1 or R11 and, if the resistors R1 and R11 are replaced with constant-current sources, the S/N ratio is degraded. In particular, a good S/N ratio is important to the mixer circuit, and it is unfavorable to replace the resistor R11 by a constant-current source.
An automatic gain control circuit, an attenuator circuit, a clamp circuit and the like can be utilized in order to prevent an operation current from increasing even when a high-level signal is input. These circuits are however complicated and, when they are inserted into a signal path, the S/N ratio deteriorates. Thus it is not advisable to employ the circuits.